Aerial navigation seeks to avoid turbulent atmospheric zones. To detect them and anticipate them, aircraft generally employ a meteorological radar operating with centimetric waves.
The principle of radar detection of dangerous zones relies notably on the analysis of the radar echoes of tracers which are generally hydrometeors such as for example drops of water, snow, hail or supercooled ice. In proximity to the ground, it is also possible to utilize the reflection of the radar waves on non-aqueous meteors such as dust and insects for example, these elements being entrained by the movements of the air mass. However, at high altitude, only hydrometeors can be utilized.
When turbulence zones that might lie on the route of the aircraft contain such tracers, the customary procedures rely:                either on the measurement of a very significant radar reflectivity, greater than about 45 dBz, which characterizes the presence of strong precipitations;        or on the measurement of a significant Doppler spreading of the radar echoes which conveys the presence of significant haphazard motions of the meteors entrained by the surrounding mass.        
There exist, however, particular flight configurations where significant turbulence arises at spots devoid of any tracer that might produce radar echoes. Such “clear sky” turbulences are known by the acronym CATs standing for the expression “Clear Air Turbulences”. One of these particular situations may be encountered when overflying stormy convective systems such as cumulo-nimbus notably. In this case, the customary procedures do not allow the detection of turbulence. A known solution could use radar systems of decimetric to decametric wavelengths which utilize notably the Bragg diffraction due to the slight variations in the refractive index of the atmosphere when the latter is the seat of turbulent phenomena. These systems are, however, hardly compatible with installation on an aircraft on account notably:                of the size of the aerials required, in particular because of the large wavelengths involved;        of the transmission power required to obtain sufficient range, therefore advance notice, having regard to the weakness of the reflectivity resulting from the Bragg diffraction phenomenon.        